Ken, replies below. Regards, Keith Armstrong In a message dated 06/01/02 06:56:46 GMT Standard Time, ken.ja...@emccompliance.com writes:
> Subj:Re: EMC-related safety issues > Date:06/01/02 06:56:46 GMT Standard Time > From: ken.ja...@emccompliance.com (Ken Javor) > To: cherryclo...@aol.com, cortland.richm...@alcatel.com > CC: emc-p...@majordomo.ieee.org > > What an EMC engineer who understands the physics of field-to-wire coupling > would say is that the operation of non-antenna connected electronics > associated with one subsystem will not be degraded by close proximity with > the non-antenna connected electronics of another subsystem. Forget 10 > meters. Are the PCs in your office separated by 10 m? Would you expect > two PCs stacked side-by-side or one on top of the other to interact in any > manner? These are rhetorical questions. > I don't see the relevance of this paragraph to my example. This might be because I am a little slow. I would appreciate more explanation. > About the blood pressure monitor example. Not enough info here to back out > what is wrong, but basic logic theory says when the conclusion is > impossible, you must re-examine your assumptions. If 92 dBuV/m were enough > to make the device malfunction, it would malfunction a lot and there would > have been enough trouble reports to get it fixed or withdrawn. And 92 > dBuV/m at 10 meters is SCREAMING!!! I am on location and don't have FCC > regs easily available, but the limits stair-step around 40 dBuV/m at 3 m, > per my recollection. This is exactly why I emphasised that the front-panel display of the blood sample incubator (not a blood pressure monitor) continued to read the set-point temperature (37.1C) even though the actual temperature could be way off. I am confident that in actual operation the incubator temperature was quite often at least a few degrees C different from what was displayed on its front panel, probably affecting the performance of the reagents. And I don't think that 92dBuV/m is a high field strength to be emitted by a PC placed nearby, or for a non-compliant laptop at 10 metres. > Would I feel comfortable placing a CISPR compliant PC next to a medical > device qualified to 1 V/m? There is an inherent (not planned) margin of > safety here that is many orders of magnitude. The answer is absolutely > yes. If there were a problem, I would expect it more to occur below 30 > MHz, at the power supply switching frequency, IF the medical device > processed extremely low levels of electrical signals and was poorly > shielded. But I believe there are separate immunity requirements which > cover this eventuality as well. > Maybe when you wrote the above you weren't thinking of the previous correspondence in this thread about the proximity of the low-energy lamp to a bedside radio. Yes, I know, this concerned a radio receiver, what I mean to draw your attention to is the discussion about the intention and validity of the EMC standards – they simply do not cover situations where devices are placed close to each other – so they cannot be relied upon to provide compatibility in such situations. Military EMC standards are more thorough in this respect. And as I have already said, commercial EMC standards were not written with safety issues in mind, and most safety standards have not been written with EMC-related issues in mind (see my IEEE 2001 EMC Symposium paper and my longer article in ITEM UPDATE 2001 for details). So I cannot see that there is any 'inherent' margin of safety in the above situation, as you claim there "absolutely is". (And may I suggest that anyone who thinks that the statement in all (or most) IEC EMC immunity standards: "Products shall not become unsafe as a result of these tests." means that that products which pass those immunity tests are necessarily free from EMC-related safety problems, needs to think a little bit harder about the subject?) Regards again, Keith Armstrong > on 1/5/02 12:23 PM, cherryclo...@aol.com at cherryclo...@aol.com wrote: > > >> Dear Cortland >> People can't simply say: "ordinary semiconductors won't demodulate RF >> levels produced by an unintentional radiator" ­ even the smallest >> amount of RF can be demodulated ­ there are no hysteresis or threshold >> effects in a PN semiconductor junction or FET that is biased into its >> conduction region (at least not until you get below signal levels >> equivalent to less than a single electron). >> >> What I am sure most engineers would really mean to say is: >> "ordinary semiconductors exposed to RF levels from an information >> technology product which is fully compliant with all relevant EMC >> emissions standards and is at 10 metres distance will generally not >> demodulate a sufficient level of interference to make an appreciable >> difference to most electronic systems." >> >> Now we have a statement which has some scientific rigor and some >> engineering validity to it. >> (Although I do worry that in Europe our harmonised EMC standards only test >> emissions up to 1GHz, so what does that say about the possible emitted >> fields strengths from a PC with a 1.2GHz clock frequency?) >> >> Let's see if we can put some meat into this discussion with a real-life >> example... >> >> I once tested a blood sample incubator for RF field immunity. The >> incubator was used during screening programs (for cancer and other >> diseases) and kept about 100 test tubes at 37.1C (normal blood >> temperature), while the reagents in the test tubes changed colour. After >> 24 hours of incubation medical staff would inspect the test tubes and >> write letters to people telling them they were sick, or that they were >> clear of the disease. I don't know what temperature tolerance the reagents >> had to give an accurate medical diagnosis, so assume ±0.1C. >> >> On the front panel of the incubator was a display of its temperature, >> which was of course 37.1C. We found that field strengths as low as 1V/m >> would cause the incubation temperature to range over full scale, from >> heaters fully off (in which case the temperature would decline to ambient) >> to maximum (in which case the water used to incubate the test tubes would >> boil). >> We could use the RF test frequency to control the temperature between plus >> and minus full scale over the frequency range 80 to 1000MHz at 1V/m (and >> did not test beyond 1GHz). >> >> Most worryingly, the front panel display would only show temporary >> variations from its 37.1C when the RF field was turned off or on, and >> would continue to show 37.1C even when the water in the incubator was >> stone cold or actually boiling. >> >> Most demodulation effects in bipolar and FET devices approximate to a >> square law - for example a 1dB fall in the field strength (keeping >> everything else constant) would typically result in a 2dB fall in the >> demodulated 'interference' error signal, as John Woodgate has recently >> pointed out. >> >> If we assume that the 1V/m field strength was causing a 60C temperature >> error, how low would we need to make the RF field to get down to the 0.1C >> accuracy of the front panel display? >> >> Assuming square-law characteristics for the device doing the demodulation >> I calculate a field strength of around 40mV/m or 92dBmicrovolts/metre. >> >> You will notice that I have been generous to the incubator and assumed >> that the 1V/m field just about caused its temperature error to increase by >> 60C to boil the water, whereas it could have been overdriving the internal >> circuits by a considerable margin and still suffered a 60C error at >> 0.1V/m. We didn't test this possibility as our focus was (as in most of >> these cases) on quickly modifying the product so it passed the immunity >> test - which we did.. >> >> 92dBmicrovolts/metre is not a very high RF field level for a PC without >> any EMC precautions at a distance of 10 metres. >> >> How many people reading this would be now be quite happy to place even a >> fully-compliant PC (compliant at 10 metres distance, that is) right next >> to the unmodified incubator? >> >> If it helps, imagine that it is your young daughter whose blood sample is >> in the incubator to discover which drugs she needs to survive. >> >> Shall we have a vote on how close we would be prepared to place the PC? >> Might be interesting. >> >> Let's not even think about the problems of proximity to cellphones and >> other intentional radiators. >> >> I didn't mention that the incubator was a small model used for mobile >> screening, for installation in a truck adapted for medical screening >> purposes which travels to various communities and parks there for a few >> days while it tests the local people for disease - hardly a very well >> controlled electromagnetic environment. >> >> What does the above imply for similar incubators in countries that do not >> have mandatory EMC immunity standards? Or for older incubators in the EU >> that have never had to meet the EMC directive? >> >> (Please don't reply with the old chestnut that "we haven't heard of any >> problems so far, so everything must be OK" - people who should have known >> better were using that phrase before September 11th. It is just not an >> acceptable argument where safety issues are involved, as any expert in >> safety law will tell you. Try: "I've been driving past that school at >> 40mph for ten years and haven't hit a kid yet, so it must be safe mustn't >> it?" as a test of the concept.) >> >> Regards, Keith Armstrong >> >> In a message dated 04/01/02 17:54:23 GMT Standard Time, >> cortland.richm...@alcatel.com writes: >> >> >>> Subj:Re: EMC-related safety issues >>> Date:04/01/02 17:54:23 GMT Standard Time >>> From: cortland.richm...@alcatel.com (Cortland Richmond) >>> To: cherryclo...@aol.com >>> CC: emc-p...@majordomo.ieee.org >>> >>> I don't believe this is what people are saying here. What they are saying >>> is, ordinary semiconductors won't demodulate RF levels produced by an >>> unintentional radiator. Cortland >>> (What I write here is mine alone. >>> My employer does not >>> Concur, agree or else endorse >>> These words, their tone, or thought.) >>> >>> cherryclo...@aol.com wrote: >>> >> Does anyone else think that ordinary semiconductors doesn't respond to RF? >> I have tested a product which was little more than an LM324 quad op-amp >> for RF immunity using IEC 61000-4-3. This op-amp has a slew rate of >> 1V/micro-second on a good day with the wind in its favour. It was housed >> in an unshielded plastic enclosure. Demodulated noise that exceeded the >> (not very tough) product specification were seen all the way up to 500MHz >> at a number of spot frequencies that appeared to be due to the natural >> resonances of the input and output cables. >>
